Sophisticated electrified rapid transit rail systems have been put into operation which provide high speed and maximum safety features. Such systems traditionally have a third rail which carries propulsion current and the running, or traction, rails are used as the return path for the propulsion current. For reasons which are understood by those skilled in the art, but which need not be explained further for the description of the present invention, it is desirable to assure that the traction rails carry approximately equal values of return propulsion current. In order to provide equalized current in the traction rails, it is necessary to provide low impedance electrical bonds between the rails at periodic intervals. The General Railway Signal Company of Rochester, New York provides bonds which are appropriate for this purpose and provide a wide variety of other useful features. The General Railway Signal Company bond is marketed under the name Wee-Z Bond. In addition to conducting return propulsion current, the traction rails are also used to transmit a variety of other signals which may convey information relating to allowable speed and other train controls. The rail bonds must not interfere with the other signals in the track. The General Railway Signal Company Wee-Z Bonds between the rails serve at least the following functions:
1. Equalize propulsion return currents between the traction rails.
2. Provide a means to cross-bond one track to a parallel track.
3. Provide a means to return the propulsion current to a substation.
4. Define the end boundaries of track circuits.
5. Provide a means for coupling a track circuit frequency, a cab signal frequency, and sometimes a wayside-to-train (TWC) signal frequency into the rails.
6. Provide a means for coupling a received track circuit signal frequency and a train-to-wayside (TWC) signal frequency from the rails into a receive signal cable.
7. Provide a very low impedance shunt to all frequencies in the rails to which the bond is not tuned in order to stop the propogation of unwanted signal frequencies in the track.
From the foregoing, it will be obvious that many design limitations are placed on a bond and that the bond may be required to conduct substantial currents between the rails. Accordingly, these bonds are relatively expensive and bulky items, and any means for making them simpler, more economical or reducing the number required will result in substantial savings.
As indicated, a variety of communicating and control signals may be passed through the rails. It is common practice to communicate such signals as modulated signals on a carrier wave. In prior art systems, a bond of the type described above is provided at each track circuit boundary. And at each boundary, a transmit and receive unit is provided. Adjacent track sections usually use different carrier frequencies to avoid any interference. Thus, at a particular boundary point, the receiver receives frequencies of one carrier frequency from one side of the bond and transmits signals at another carrier frequency to the other side of the bond. The distance between track circuit boundaries is determined by a variety of factors, some of which relate to physical conditions such as the location of switches; the location of stations; the location of highway crossings; and other factors with which those familiar with the art are aware. In addition, the distance between track circuit boundaries may be limited by the attenuation of the signal in the track.